Use noindex meta tag or header, not robots.txt, to block untranslated docs pages

As far as I can tell, we're currently using `robots.txt` to block untranslated docs pages from being indexed by Google. Looking through Search Console, it appears that is not the correct approach and these pages are still being indexed:

That warning links to: https://support.google.com/webmasters/answer/7440203#indexed_though_blocked_by_robots_txt

Which suggests using the `noindex` meta tag or header instead: https://support.google.com/webmasters/answer/93710

No file or test is named in the issue. Start by locating how robots.txt blocks untranslated documentation pages and where those pages are rendered; verify the affected URLs use a noindex meta tag or header instead, then check that the indexing behavior is no longer blocked solely by robots.txt.

Written by the indexing model from the issue text.